The present invention relates generally to an image coding apparatus and method, an image decoding apparatus and method, and a recording medium. More specifically, the present invention relates to an image coding apparatus and method, an image decoding apparatus and method, and a recording medium which are suitable for use in apparatus for re-encoding video streams and recording and reproducing the re-encoded video streams.
Digital television broadcasts such as European DVB (Digital Video Broadcast), American DTV (Digital Television) broadcast, and Japanese BS (Broadcast Satellite) digital broadcast use MPEG (Motion Picture Expert Group) 2 transport streams. A transport stream consists of continuous transport packets, each packet carrying video data or audio data, for example. The data length of one transport packet is 188 bytes.
Unlike analog television broadcasts, digital television broadcasts are capable of providing services added with multimedia coding data. In these services, data such as video data, audio data, character graphics data, and still picture data, for example, are associated with each other for transmission by the multimedia coding data. For the multimedia coding data, a coding method based on XML (Extensible Markup Language) is used in the Japanese BS digital broadcast, for example. The details of this method are disclosed in ARIB STD-B24 Data Coding And Transmission Specification for Digital Broadcasting, for example.
Data such as video data, audio data, character graphics data, and still picture data are each packetized into a transport packet for transmission.
FIGS. 1A and 1B show an example of synthesizing data to be transferred between the sending and receiving sides and a multimedia screen. As shown in FIG. 1A, the sending side sends to the receiving side video data, character graphics data for displaying buttons A through C, text data for displaying “XYZABC . . . ,” and multimedia coding data for relating these data to each other. The sending side generally denotes a television broadcast station, for example. However, herein it denotes a television broadcast station which includes a recording apparatus (the recording side) which receives and records data transmitted from broadcast stations, as shown in the example illustrated in FIG. 1A including the data which is output from this recording apparatus.
The multimedia coding data includes data which can synthesize on the receiving side video data, character graphics data, and text data and display the synthesized data. To be more specific, the multimedia coding data includes the data associated with the display positions of the video, character graphics, and text which are displayed by the size-associated data such as the multimedia plane (the display area of images on the television receiver, for example) size (plane_height and plane_width) and video display size (video_height and video_width), video data, character graphics data, and text data, as shown in FIG. 1B.
On the basis of the multimedia coding data, the receiving side processes the video data, the character graphics data, and the text data to display a resultant image, as shown in FIG. 1B.
Through the screen on which the above-mentioned image is displayed, the user can receive services such as displaying desired information in the video section by clicking button A corresponding to that information and obtaining, from the text data displayed in the bottom of the screen, the information associated with the matter displayed in the video section, for example.
If a television program carried by a transport stream transmitted from a digital television broadcast is recorded without change to a recording medium on the received side, the program can be recorded without its picture and audio qualities being deteriorated at all. However, in order to record as long a television program as possible to a recording medium having a limited recording capacity by presupposing a certain degree of picture quality deterioration, the received video stream must be decoded and then encoded again to lower the bit rate of the transport stream.
For example, the re-encoding of the video stream of a television program attached with multimedia coding data to lower its bit rate for recording may be implemented by sub-sampling the image to change writing blocks. However, this approach presents a problem of causing a mismatch in the relationship between the video stream resulting from re-encoding and the multimedia coding data. The following describes an example of this mismatch with reference to FIGS. 2A and 2B.
In the example shown in FIG. 2A, the sending side (the recording side) converts the original video writing block to a smaller picture frame at the time of re-encoding. Therefore, as shown in FIG. 2B, on the receiving side (the reproducing side), changes occur in the video display size and position, resulting in a display screen which is different from the display screen intended by the sending side (the display screen to be displayed on the basis of the data before being re-encoded).